Ethanol (CH3CH2OH) can be used as a liquid fuel to run engines such as cars. A significant market for ethanol as a liquid fuel already exists in the current transportation and energy systems. In the United States, currently, ethanol is generated primarily from corn starch using a yeast-fermentation process. Therefore, the “cornstarch ethanol production” process requires a number of energy-consuming steps including agricultural corn-crop cultivation, corn-grain harvesting, corn-grain starch processing, and starch-to -sugar-to-ethanol fermentation. Independent studies have recently shown that the net energy efficiency of the “cornstarch ethanol production” process is actually negative. That is, the “cornstarch ethanol production” process costs more energy than the energy value of its product ethanol. This is not surprising, understandably because the cornstarch that the current technology can use represents only a small fraction of the corn crop biomass that includes the corn stalks, leaves and roots. The cornstovers are commonly discarded in the agricultural fields where they slowly decompose back to CO2, because they represent largely lignocellulosic biomass materials that the current biorefinery industry cannot efficiently use for ethanol production. There are research efforts in trying to make ethanol from lignocellulosic plant biomass materials—a concept called “cellulosic ethanol”. However, plant biomass has evolved effective mechanisms for resisting assault on its cell-wall structural sugars from the microbial and animal kingdoms. This property underlies a natural recalcitrance, creating roadblocks to the cost-effective transformation of lignocellulosic biomass to fermentable sugars. Therefore, one of its problems known as the “lignocellulosic recalcitrance” represents a formidable technical barrier to the cost-effective conversion of plant biomass to fermentable sugars. That is, because of the recalcitrance problem, lignocellulosic biomasses (such as cornstover, switchgrass, and woody plant materials) could not be readily converted to fermentable sugars to make ethanol without certain pretreatment, which is often associated with high processing cost. Despite more than 50 years of R&D efforts in lignocellulosic biomass pretreatment and fermentative ethanol-production processing, the problem of recalcitrant lignocellulosics still remains as a formidable technical barrier that has not yet been eliminated so far. Furthermore, the steps of lignocellulosic biomass cultivation, harvesting, pretreatment processing, and cellulose-to-sugar-to-ethanol fermentation all cost energy. Therefore, any new technology that could bypass these bottleneck problems of the biomass technology would be useful.
Algae (such as Chlamydomonas reinhardtii, Platymonas subcordiformis, Chlorella fusca, Dunaliella salina, Ankistrodesmus braunii, and Scenedesmus obliquus), which can perform photosynthetic assimilation of CO2 with O2 evolution from water in a liquid culture medium with a maximal theoretical solar-to-biomass energy conversion of about 10%, have tremendous potential to be a clean and renewable energy resource. However, the wild-type oxygenic photosynthetic green plants, such as eukaryotic algae, do not possess the ability to produce ethanol directly from CO2 and H2O. As shown in FIG. 1, the wild-type photosynthesis uses the reducing power (NADPH) and energy (ATP) from the photosynthetic water splitting and proton gradient-coupled electron transport process through the algal thylakoid membrane system to reduce CO2 into carbohydrates (CH2O)n such as starch with a series of enzymes collectively called the “Calvin cycle” at the stroma region in an algal or green-plant chloroplast. The net result of the wild-type photosynthetic process is the conversion of CO2 and H2O into carbohydrates (CH2O)n and O2 using sunlight energy according to the following process reaction:nCO2+nH2O(CH2O)n+nO2  [1]The carbohydrates (CH2O)n are then further converted to all kinds of complicated cellular (biomass) materials including proteins, lipids, and cellulose and other cell-wall materials during cell metabolism and growth.
In certain alga such as Chlamydomonas reinhardtii, some of the organic reserves such as starch could be slowly metabolized to ethanol through a secondary fermentative metabolic pathway. The algal fermentative metabolic pathway is similar to the yeast-fermentation process, by which starch is breakdown to smaller sugars such as glucose that is, in turn, transformed into pyruvate by a glycolysis process. Pyruvate may then be converted to formate, acetate, and ethanol by a number of additional metabolic steps (Gfeller and Gibbs (1984) “Fermentative metabolism of Chlamydomonas reinhardtii,” Plant Physiol. 75:212-218). The efficiency of this secondary metabolic process is quite limited, probably because it could use only a small fraction of the limited organic reserve such as starch in an algal cell. The maximal concentration of ethanol that can be generated by the fermentative algal metabolic process is only about 1%, which is not high enough to become a viable technology for energy production. To be an economically viable technology, the ethanol concentration in a bioreactor medium needs to be able to reach as high as about 3-5% before an ethanol-distillation process could be profitably applied. Therefore, a new ethanol-producing mechanism with a high solar-to-ethanol energy efficiency is needed.
The present invention provides revolutionary designer organisms, which are capable of directly synthesizing ethanol from CO2 and H2O. The ethanol production system provided by the present invention could bypass all of the bottleneck problems of the biomass technology mentioned above.